CAMDEN, S.C. — The feud between Ted Cruz and Donald Trump soared to new heights of animosity Monday as the billionaire and U.S. senator from Texas traded accusations of dishonesty and desperation.

Trump upped the ante with just five days until the South Carolina GOP Republican primary, issuing a new threat to sue Cruz over his eligibility to be president if “he doesn’t take down his false ads and retract his lies.”

“Ted Cruz is a totally unstable individual,” Trump said in a statement. “He is the single biggest liar I’ve ever come across, in politics or otherwise, and I have seen some of the best of them.”

Trump continued: “His statements are totally untrue and completely outrageous. It is hard to believe a person who proclaims to be a Christian could be so dishonest and lie so much.”

Cruz fired back later Monday afternoon, telling a crowd here that Trump has “apparently lost it” with his latest ultimatum. Trump repeated the threat at a news conference in the Palmetto State, where Cruz said the billionaire was the “most rattled” he has ever seen him.

“He was just going on and on about how I’m the most horrible person in the world because you know, I keep repeating things he’s said,” Cruz said, later adding with some sarcasm: “I think he avoided profanity, so that was good.”

Comparing Trump’s news conference to the billionaire’s harsh reaction to losing the Iowa caucuses, Cruz went on to say the “only explanation one could have is that his internal poll numbers for South Carolina must be plummeting.”

Cruz’s campaign continued to needle the billionaire Monday, releasing a TV ad it billed as a “response to today’s Trump meltdown.” Titled “Chance,” the 30-second spot draws heavily on a recent TV interview in which Trump described himself as “very capable of changing to anything I want to change to.”

The fracas between Cruz and Trump is unfolding two days after their nastiest meeting yet on the debate stage, where the billionaire called the senator the “single biggest liar.” For weeks now, the two have been locked in an increasingly heated battle, with Cruz casting doubt on Trump’s conservative credentials and Trump accusing Cruz of running a deceptive campaign.

Speaking with reporters Monday morning in Aiken, Cruz widened a few new fronts in the battle, using the death of U.S. Supreme Court Justice Antonin Scalia to sharply question what kind of judges Trump would put on the high court as president.

“The one person Donald has pointed to as a potential Supreme Court nominee is his sister,” Cruz told reporters. “Now it’s good to stand with your sister, but Donald’s sister was a Bill Clinton-appointed federal appellate judge who is a radical pro-abortion extremist.”

Trump’s sister, Maryanne Trump Barry, sits on the U.S. Court of Appeals for the Third Circuit. In 2000, she wrote the majority opinion in Planned Parenthood of Central New Jersey v. Farmer, a case that struck down the state’s prohibition against late-term abortions.

Cruz appeared to make reference to that decision Monday morning, saying, “even among liberal judges, that position is extreme, and Donald said his extreme, abortion-supporting sister would make a terrific Supreme Court justice.”

In an interview last year with Bloomberg Politics, Trump said he thought his sister would be a “phenomenal” choice for the Supreme Court. In another interview Sunday, this one on ABC News’ “This Week,” Trump said he was kidding when he suggested her for the high court, adding that she “happens to have a little bit different views than me.”

In any case, Cruz kept Trump in his crosshairs throughout the day, especially on the topic of judicial appointments. In the wake of Scalia’s death Saturday, Cruz has urged voters to consider which candidate they would most trust to pick conservative jurists for the high court.

“Donald Trump, if he were president — he would appoint liberals to the Supreme Court,” Cruz told reporters in Aiken. “I know this for a fact. Why? Because Donald has been a liberal his entire adult life.”

In any case, Cruz’s remarks are likely to raise the temperature between himself and Trump five days before the first-in-the-South primary in South Carolina. In the wake of Scalia’s death Saturday, Cruz has asked voters to consider which candidate they most trust to appoint conservative jurists to the U.S. Supreme Court.

Cruz also zeroed in on Trump’s criticism at the debate of former President George W. Bush for his decision to invade Iraq. Trump has previously expressed some preference for impeaching Bush over the War an Iraq, which Cruz called an “extreme and radical position” that aligns him with some of the most liberal voices in the Democratic Party.

“When Donald Trump sided with MoveOn.org and Michael Moore and the extreme fever-swamp left wing, on calling for the impeachment of George W. Bush, that demonstrated where he was coming from,” Cruz told reporters.

PATRICK SVITEK reports for The Texas Tribune where this story was originally published. It is made available here through a news partnership between the Texas Tribune and the San Marcos Mercury.